1. Field of the Invention
The present invention relates to a particle therapy system designed for radiotherapy.
More particularly, the present invention concerns a particle therapy system for irradiating with a particle beam a plurality of spots in a layer of a target, said system comprising means for scanning the particle beam over said layer a prescribed number of layer scanning times (Nlayer_painting), and for irradiating selected spots of said layer with partial spot doses (Dspot_partial) in the course of each said layer scanning time so that each spot respectively receives a prescribed total spot dose (Dspot) after completion of the prescribed number of layer scanning times, said system being only able to irradiate a spot with a spot dose which is larger than or equal to a minimum spot dose (Dmin).
Dmin is the smallest dose which the therapy system can deliver to a spot of the target with an accuracy that is adequate for particle therapy. Such minimum spot dose (Dmin) is a feature of the particle therapy system which originates from its particular design. It may for example originate from its dosimetry equipment which has a specific operating range where the accuracy of dose measurement is guaranteed, or from its beam position detector which needs a minimum number of statistically relevant particle counts to determine the position of the beam with the required accuracy, or form limitations with respect to the delivery and control of the particle beam intensity.
The invention also concerns a method for irradiating a target with a particle beam.
2. Description of Prior Art
Such a particle therapy system is known for example from M. Phillips et al. in “Effects of respiratory motion on dose uniformity with a charged particle scanning method”, Phys. Med. Biol., 1992, vol 37, No 1, 223-234 or from Pedroni et al. in “A novel gantry for proton therapy at the Paul Scherrer Institute”, Cyclotrons and their applications 2001.
As discussed by Phillips et al., the use of scanned beams for irradiating a target can, due to organ motion, result in “cold” and “hot” dose areas. They showed that when a layer of the target volume is scanned (sometimes also called “painted”) a plurality of times and when spots are irradiated with partial doses in the course of each scanning (“painting”) time, the dose uniformity improves roughly with the square root of the number of scannings (“paintings”).
Similarly, also Bortfeld, et al. in “Effects of intra-fraction motion in IMRT dose delivery: statistical analysis and simulation”, Phys. Med. Biol. 47 (2002), pages 2203-2220, have studied the effect of organ motion on the dose uniformity when using a multiple painting spot scanning technique.
With the therapy systems disclosed by both Phillips et al. and Bortfeld et al., the partial spot dose of a spot (Dspot_partial) is obtained by dividing the prescribed total irradiation dose of said spot (Dspot) by the number of times the target is to be scanned/painted (Nlayer_painting). Hence, at each scanning time, a spot will be irradiated with the same partial dose.
However, Phillips et al. and Bortfeld et al. do not provide any information on what range of values should be selected for Nlayer_painting and they do not take into account the influence of a particular value of Nlayer_painting on the feasibility or accuracy of the delivery of the partial doses by a real particle therapy system. In particular, the particle therapy system will not be able to deliver partial spot doses which are smaller than the minimum spot dose (Dmin) or it will not be able to deliver them with sufficient accuracy.
This problem becomes even more relevant in case the distribution of the prescribed total spot doses is non-homogenous, i.e. when some spots of the target have a high and others a low prescribed total dose, which is often the case with spot scanning systems. The case being, when applying the teaching of Phillips et al. and Bortfeld et al., the number of layer paintings would logically be set to a relatively large value in order to reduce the effects of organ motion and, hence, this will increase the risk that the partial spot doses will fall below the minimum spot dose Dmin for those spots having a low prescribed total spot dose.
Patent document EP1477206 also discloses such a known particle therapy system, though no organ motion problems are addressed in this document. Instead, EP1477206 addresses a problem related to the inaccuracy of dose delivery that is caused by limitations of a capacitor used by a dose detector of the system. To solve this problem, this document discloses a particle therapy system wherein the number of layer paintings are set to a value which depends on a reference spot dose, which itself depends on the design of the apparatus, such as the design of the dose monitoring system. Such reference dose is a dose which this system can effectively deliver with a required accuracy.
With such a system, the partial spot doses are set to a value which is close to said reference dose, but the drawback is that the number of layer paintings cannot be freely chosen. Hence, one cannot optimize the number of layer paintings on the basis of other criteria such as for the reduction of effects for organ motion, or for reducing the treatment time.
There is thus room for improvement of the known particle therapy systems.